1. Field of the Invention
This invention relates to apparatus for controlling the fluid level in, between and when discharging liquid from flotation cells, or tanks, which are used for separation of liquid and solid phases of an influent fluid. Specifically, this invention relates to fluid control apparatus which operates on a reduced scale, and with a specially designed valve to provide more controlled flow of fluid.
2. State of the Art
Flotation cells or tanks are widely used in a variety of industries, such as oil, wastewater treatment, pulp and paper, and mining and mineral reclamation, to separate an influent liquid or feed slurry into a clarified liquid phase and a solid or particulate matter phase. Flotation tanks generally operate by facilitating the flotation of solid or particulate matter, such as mineral-bearing particles, to the top of the fluid bed contained in the tank, while a liquid phase develops toward the bottom of the tank. The liquid phase may typically contain varying amounts of solids or particulates which are not completely separated from the liquid. Thus, the liquid phase may range from a relatively clarified liquid to a pulp or slurry. Flotation of the solid phase or mineral particles to the top of the tank is often facilitated by such means as introducing air into the influent liquid to form a froth which captures or binds the solid phase or mineral particles and floats them to the top of the liquid volume in the tank. The solids or concentrated mineral particulate matter which have been floated to the top of the liquid level are typically removed from the tank by causing the floating material to overflow into a launder, usually positioned about the periphery of the tank.
It is important for the efficient operation of flotation cells that the liquid level in the tank be maintained within a certain specified range so that the floating mineral concentrate, or froth bed, also remains at a specified level in the flotation cell to optimize recovery of the solid or particulate matter. In other words, if the liquid level in the flotation cell is too low, the separated solids or mineral concentrate, also referred to herein as “the float,” will remain afloat on the liquid volume and will not overflow into the launder, thereby increasing the residency time of the float. The longer the float stays in the tank, the greater the possibility that the solid or mineral concentrate will sink back into the liquid volume and decrease the efficiency of the separation process. Conversely, if the liquid level in the tank is too high, the float may move efficiently to the overflow launder, but an increased amount of liquid will overflow and enter the launder as well. An inordinate amount of liquid in the overflow launder reduces the efficiency of the later processing of the mineral concentrate passing along valuable material which goes out with the tails.
Thus, it has been recognized for some time that it is beneficial to the operation of flotation tanks to provide means for controlling the liquid (i.e., pulp or slurry) level in the tank. Control devices which are conventionally used in the industry comprise a separate tank, often termed a “box,” which is positioned externally to the flotation cell. The control box is in fluid communication with the flotation cell via one or more conduits interconnected between the flotation cell and the control box. In large plant operations, control boxes are typically interconnected between two adjacent flotation cells and are in fluid communication with both flotation cells. Conventional control boxes generally include a valve positioned internally to the box which operates to let fluid flow through the box from one flotation cell to the next adjacent flotation cell, thereby modifying the liquid level in both flotation cells. It is noted that, while discussed in terms of controlling the flow between two adjacent flotation cells, the control box may be used to control the fluid level in a single cell if so desired.
Conventional fluid level control boxes tend to be substantially the same height as the flotation cell since the liquid level in the control box is maintained at approximately the same depth as the liquid in the flotation cell. Thus, for example, the control box may range in height from five feet to twenty feet. The length of conventional control boxes may generally be just short of the diameter of the flotation cell (e.g., three to six feet or greater) and may be one to five feet wide. In many large industrial applications (including, for example, mineral applications), several flotation cells are positioned adjacent each other and are all placed in fluid communication with one or more adjacent flotation cells so that the liquid flow from one flotation cell is directed to the next adjacent cell, and so on. Conventional liquid level control boxes are positioned between adjacent flotation cells so that the liquid flowing from a first flotation cell enters into the control box. Liquid then enters into the next adjacent flotation cell through a conduit interconnected between the control box and the second flotation cell. By so arranging the flotation cells and liquid control boxes therebetween, the liquid level in each individual cell of a grouping of flotation cells can be optimally controlled.
One of the major drawbacks encountered with use of conventional liquid control boxes is their size, which not only increases capital costs in operation of the flotation cells, but limits the area capacity and, therefore, the number of flotation cells which may be installed at a given plant site. That is, conventional fluid control boxes are so large, and must necessarily be located between adjacent flotation cells, that they take up vital space which may be used for the installation of more flotation cells or which may be used for other purposes. Furthermore, when maintenance is required on conventional control boxes, the flotation cells to which the control box is attached must be taken off-line while repairs are effected.
Additionally, the control valves of known liquid control boxes are, by virtue of their configuration, unable to provide finely controlled release of liquid through the control box. More specifically, known control valves provide an initial rapid flow rate of liquid which levels off quickly as the valve is opened. The fluid flow dynamics of conventional fluid level control boxes are, therefore, less subject to finite control.
It would be advantageous, therefore, to provide a fluid level control apparatus which provides finely controlled fluid flow therethrough, thus providing ease of maintenance and repair, and which reduces capital costs by reducing the size and operation of the liquid level control apparatus and by enabling more flotation cells to be installed at a plant site.
It would be further advantageous to provide a system of flotation cells wherein fewer valves are used wherein the liquid level to which a given cell is set is controlled within a relatively tighter range due to the reduction of valves being used.